Phase constant of SHM

The displacement of a mass oscillating on a spring is given by x(t) = xmcos(ωt + ). If the initial displacement is zero and the initial velocity is in the negative x direction, then the phase constant is:

2. Relevant equations

3. The attempt at a solution

How do I start? The book just tells me that the phase constant depends on displacement and velocity when t = 0, but doesn't say how.